The present invention relates generally to thermophotovoltaic devices for generating electrical energy, and more particularly to the process of fabricating an interdigitated multicell device that is particularly efficient for generating electrical energy from a heat source by operation in the near field. In this and similar applications, a very high efficiency device is required that absorbs more in band than out of band photons. Inefficient thermophotovoltaic cells cause a drop in the temperature of the emitter used to form near fields and this results in a poorer photon transmission to the thermophotovoltaic cell.
Photovoltaic energy conversion is a direct conversion process that transfers electromagnetic energy in the form of photons from an emitter to a photovoltaic device for generation of electrical energy by the receiving photovoltaic device. The most widely recognized photovoltaic devices are based on semiconductor technology and optimized for operation in the solar spectrum, i.e. light from the sun, while less well-known semiconductor photovoltaic devices that are optimized for operation in the infrared and near-infrared spectrum are referred to as thermophotovoltaic devices. For operation in the solar spectrum, photovoltaic devices generate electrical energy by absorbing photons of energy in the 1.0 to 5.0 electron volt range. Thermophotovoltaic devices generate electrical energy by additionally absorbing lower energy photons in the 0.2-1.0 electron volt range. To provide the best match for a thermophotovoltaic device and the incoming spectrum of photon energy from a low temperature heat source requires a narrow bandgap of the semiconductor material of the thermophotovoltaic device. Photons having energy greater than or equal to the semiconductor material bandgap can generate electrical energy, while lower energy photons generate heat and result in a loss of efficiency. In addition, photon energy well in excess of the semiconductor material bandgap is also partially lost as heat.
Thermophotovoltaic devices comprise one or more semiconductor P-N junctions or diodes that collect and separate the electron/hole pairs that are generated by the absorption of photon energy and thereby produce electrical energy. The characteristics of a thermophotovoltaic device within a thermophotovoltaic system provide opportunities for optimizing the operation of that system through use of additional components such as concentrators, filters, reflectors and selective emitters. Multiple thermophotovoltaic devices may also fabricated on a single substrate for large array applications.
Thermophotovoltaic devices receive photon energy from a relatively hot emitter separated from a relatively cool receiving thermophotovoltaic device by a gap. When the gap spacing between an emitter and receiver is one micron or less, which is considered to be near field operation, greater power transfer is achieved than that predicted by Planck's Law for black body radiation for far field operation. For gap spacing of one-tenth micron, energy transfer increases by factors of five or more are possible when compared with that predicted by far field theory. However, such narrow gap spacing generally requires a vacuum in a gap between a relatively hot emitter and a relatively cool thermophotovoltaic device to reduce the effects of heat conduction. Although increases in energy transfer between an emitter and a thermophotovoltaic device receiver may be achieved by increasing the temperature of the emitter, material limitations place a practical limit on a maximum temperature of operation of these devices. Also, at higher device temperatures, intrinsic carrier generation within the semiconductor device prevents effective collection of electrons.
Although in theory any material that can support the temperature can be used as an emitter, certain advantages, such as a more favorable output spectrum, may pertain to selective materials. Although not limited to these examples, materials used for, emitters in thermophotovoltaic systems include: single crystal silicon, polycrystalline silicon, silicon carbide, tungsten, rare-earth oxides and photonic crystals. Thermophotovoltaic devices may be fabricated from materials such as silicon, germanium, gallium antimonide, indium gallium arsenide antimonide, indium gallium arsenide, and indium phosphide arsenide antimonide.
Previous fabrication methods for producing thermophotovoltaic devices have employed thin active layers comprising multiple narrow cells on a window-like substrate. These isolated cells are interconnected in series for producing a higher voltage at lower output current levels in order to minimize power loss. Increased production costs result from the additional complexity of fabricating multiple cells on a common substrate.
These previous devices also employ a collection method where electron-hole pairs created from incoming photons travel perpendicular to the plane of the device to reach their respective collecting regions. For purposes of distinction, we shall call this situation perpendicular collection. Perpendicular collection methods are often the only option that can be employed in prior multicell devices because many of the low bandgap materials require junction formation by epitaxy, as the common methods of diffusion and ion implantation have not proved successful to date. If epitaxy alone is used, this inherently gives a perpendicular construction. A low bandgap material is required to collect predominantly low energy infrared and near-infrared photons emitted from relatively low temperature sources.
A perpendicular collection method also requires a lateral conduction layer (LCL) for conduction of photocurrents from one cell to the next cell of a multicell device. The use of perpendicular collection layer methods and lateral conduction layers requires trade-offs. A thick, heavily doped layer is desirable to minimize resistive losses in the lateral conduction layer but a lightly doped region is desired to minimize absorption of below-band photons, which can only be converted into heat.
The prior art photovoltaic devices have been designed for far field operation and have focused on obtaining the maximum output from the impinging spectrum to obtain an attractive overall system cost. The influence of near field operation is now illustrated for the case where germanium is used as the collecting semiconductor material. The maximum output consideration requires a thick germanium layer of at least 150 microns. However, such a thick layer also creates severe problems in forming a multicell device which requires physical isolation between the unit cells making up the device.
Another factor promoting thick absorbing layers of germanium in prior devices is the fact that germanium has two band gaps; a direct bandgap at 0.80 eV and an indirect gap at 0.67 eV. Indirect gaps have smaller absorption coefficients so that if one is trying to maximize photon collection for both gaps a thick absorption layer is required.
Some of the prior art multicell structures also have unresolved issues such as ohmic contact caused by the doping concentration compromise required in the LCL. Minimizing contact resistance for these devices requires elaborate and complex processes, such as tunnel junctions, which make these cells less competitive cost wise for an energy conversion system.
The epitaxial only construction of prior art devices results in the uppermost region being uniformly heavily doped across the entire face of the device in order to make good ohmic contact where contacted by metallization. This gives an excessive area of heavy doping which results in high below band absorption in this layer similar to the case with the LCL.
The distinction “epitaxial only” is used because when all of the regions are formed by epitaxy the collection configuration must be perpendicular. If at least one of the regions can be formed by diffusion or ion implant then a lateral collection construction may be possible if the diffusion length allows for reasonable geometries. Again, for distinction, we define a lateral collection method as one where the minority carrier flow to the collecting regions is parallel to the plane of the photovoltaic device.
Also, the heavily doped upper layer interferes with the ability to make a high efficiency back surface reflector because heavily doped regions do not make the best reflectors. An efficient back surface reflector is a very important component in an efficient near field energy conversion system.